1. Field of the Invention
The present invention relates to a high voltage power supply system capable of producing a stable, high voltage suitable for an X-ray tube and further producing a predetermined X-ray dose within a short repetition period.
2. DESCRIPTION OF THE PRIOR ART
A high voltage to be applied to an X-ray tube (to be referred to as "a tube voltage" hereinafter) must be stable and maintained at set values (e.g., an exposure time) during X-ray irradiation in order to obtain better image quality.
For this purpose, using high power semiconductors, DC-to-DC converter and DC-to-AC inverter techniques have recently been applied to the X-ray power supply control field.
Typically, a conventional high voltage power supply system including a DC-to-DC converter and a DC-to-AC inverter has the following arrangement.
The DC-to-DC converter is connected to a DC low voltage power source of, e.g., 300 V. The DC-to-DC converter includes a switching transistor, which switches this DC low voltage input at a predetermined switching frequency (e.g., about 10 kHz). The switching frequency is controlled by a switching control signal applied to the base of the switching transistor.
The DC-to-DC converter supplies an interrupted DC current (pulse current) at a low voltage to a DC-to-AC inverter provided at the following stage as an input DC voltage. The DC-to-AC inverter includes two transistors which, for example, are push-pull connected to each other. The primary winding of a transformer for generating an extra high voltage is connected to these transistors as collector loads. Complementary switching control signals, which are phase-controlled not to be ON at the same instant, are respectively applied to the bases of these transistors.
As a result, the push-pull connected transistors alternately repeat ON states, and an induced high voltage (e.g., 10 kV to 50 kV) is produced from the secondary winding of the transformer. The high voltage is produced at a predetermined radio frequency, which is determined by the switching frequency of the transistors of the DC-to-AC inverter.
As is well known, there are two types of X-ray irradiation operations. That is, the X-ray irradiation is repeated for a short cycle (e.g., an X-ray CT), and it is performed only once with a long period, on the order of milliseconds (e.g., a normal X-ray fluoroscopy).
The drawbacks of a conventional system according to the X-ray fluoroscopy will now be briefly outlined.
Normally, in a DC-to-DC converter of the above-mentioned type, a charging capacitor is connected to the load side of the collector, and a discharging resistor is connected in parallel with the capacitor. The capacitor and the resistor constitute a smoothing, or filtering circuit of the DC-to-DC converter. As is well known, the duty ratio of a switched DC voltage can be changed under the control of the switching control signal supplied to the base of the switching transistor of the DC-to-DC converter. Therefore, the DC output voltage of the DC-to-DC converter can be changed, depending upon the duty ratio.
X-ray generation is stopped immediately after a set irradiation time has elapsed. This can be achieved by immediately stopping the application of the driver voltage to the switching transistors of the DC-to-DC converter and the DC-to-AC inverter. However, a residual charge remains in the capacitor of the filtering circuit, and the charge is gradually discharged by the resistor. In this case, the switching transistor is turned off (open circuited). The resistance of the resistor is normally set to be high (otherwise, the load side of the transistor is undesirably short-circuited), and the time constant of the circuit thereby becomes long, say from 1 to several seconds. As a result, even after the X-ray irradiation is stopped, a residual voltage due to a residual charge can remain in the circuit for such a relatively long period of time. Therefore, when the next X-ray irradiation starts while the residual charge still remains, a tube voltage higher than a desired value is accidentally generated. In the worst case, such an abnormally high voltage may exceed an allowable tube voltage of the X-ray tube. This may cause damage to or destruction of the X-ray tube. In addition, this may cause an X-ray dose during a succeeding X-ray irradiation to be higher than a desired value.
This adversely influences X-ray image quality and even may cause medical injury to the patient under examination.
The present invention has been made in consideration of the above situation, and has as its first object to provide an X-ray high voltage power supply system which can quickly discharge the output capacitor of the DC-to-DC converter after X-ray irradiation is completed.
A second object of the present invention is to provide a safe high voltage power supply system, which can obtain a better quality X-ray image and can apply a stable tube voltage of a desired value to an X-ray tube during successive X-ray irradiations, to prevent an excess tube voltage from being generated when an X-ray imaging operation is repeated within a relatively short cycle.